1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a transmission apparatus and method for scheduling retransmission in the next generation mobile communication system having both Automatic Retransmission reQuest (ARQ) and Hybrid Automatic Retransmission reQuest (HARQ).
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a 3rd generation asynchronous mobile communication system that is based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), both being European mobile communication systems, and uses Wideband Code Division Multiple Access (CDMA).
The 3rd Generation Partnership Project (3GPP) is the entity in charge of UMTS standardization. It is now reviewing Long Term Evolution (LTE) as the next generation mobile communication system of the UMTS system. LTE, aimed at deployment in about 2010, is considered state of the art for implementing 100 Mbps high-speed packet based communication. To this end, several schemes are under discussion, those schemes include a scheme for reducing the number of nodes located in a communication path by simplifying the network configuration, and a scheme for maximally approximating wireless protocols to wireless channels. As a result, it is expected that the LTE configuration will change from the existing 4-node configuration to a 2-node or 3-node configuration.
FIG. 1 illustrates the configuration of an Evolved UMTS mobile communication system to which the present invention is applicable.
Referring to FIG. 1, as illustrated, Evolved Radio Access Networks (E-RANs) 110 and 112 are simplified to a 2-node configuration of Evolved Node Bs (ENBs or Node Bs) 120, 122, 124, 126 and 128, and Evolved Gateway GPRS Serving Nodes (EGGSNs) 130 and 132. A User Equipment (UE) 101 accesses an Internet Protocol (IP) network 114 by E-RANs 110 and 112.
ENBs 120 to 128, nodes corresponding to the existing Node Bs, are connected to UE 101 over a wireless channel, and performs a complex function, unlike the existing Node Bs.
In LTE, all user traffics including real-time services such as Voice over IP (VoIP) are serviced over a shared channel. This means that there is a need for an apparatus of collecting status information of UEs and performing scheduling depending thereon, and the scheduling is managed by ENB 120 to 128. For convenience, the terms ‘ENB’ and ‘Node B’ will herein be used interchangeably.
Like High Speed Downlink Packet Access (HSDPA) or Enhanced Dedicated Channel (EDCH), LTE also performs Hybrid Automatic Retransmission Request (HARQ) between ENB and UE. However, with use of only HARQ, LTE cannot meet various Quality of Service (QoS) requirements. Therefore, outer-ARQ is performed even between UE 101 and ENBs 120 to 128. The outer-ARQ means a normal ARQ being performed in RLC layer.
It is expected that LTE will use Orthogonal Frequency Division Multiplexing (OFDM) as wireless access technology in a 20-Mhz bandwidth in order to realize the maximum data rate of 100 Mbps. In addition, Adaptive Modulation & Coding (AMC) scheme determining the modulation scheme and the channel coding rate according to the channel status of the terminal will be applied to LTE.
Many mobile communication systems now under discussion, including LTE, use both HARQ and ARQ as error correction technique. HARQ refers to a technique of soft-combining previously received data with retransmitted data, without discarding the previously received data, thereby increasing a reception rate. More specifically, an HARQ receiver checks presence/absence of an error in a received packet, and then sends an HARQ Acknowledged (ACK) signal or an HARQ Non-Acknowledged (NACK) signal to a transmitter according to the error check result. The transmitter then performs retransmission of the HARQ packet or transmission of a new HARQ packet according to the HARQ ACK/NACK signal. That is, the HARQ technique soft-combines the retransmitted packet with the previously received packet, thereby reducing an error rate.
However, ARQ refers to a technique of checking a sequence number of a received packet and sending a retransmission request for a failed packet (or a missing packet), if any, and the ARQ technique does not soft-combine the previously received packet with the retransmitted packet.
It is argued that since both ARQ and HARQ perform the function of recovering failed packets, there is no need to use them together. However, because it is hard to obtain a sufficiently low packet error ratio only with HARQ, ARQ and HARQ should both be used for most packet services. This is because HARQ can hardly reduce the error rate through channel coding, as the HARQ ACK/NACK signal is a 1-bit response signal. That is, if an HARQ NACK signal is misrecognized as an HARQ ACK signal, the corresponding packet is completely lost at the HARQ level. Therefore, reliability of the HARQ ACK/NACK signal serves as an important factor for determining the packet error ratio in the HARQ level. Thus, in the mobile communication systems using both HARQ and ARQ, with use of HARQ transmission information, an ARQ transmitter can perform fast retransmission. However, in the current mobile communication systems, there is no discussion on a detailed method in which the ARQ transmitter performs retransmission using the HARQ transmission information. Therefore, there is a need for a detailed method for preventing the ARQ transmitter from performing unnecessary retransmission.